1. Field of the Invention
The present invention relates to a control unit for a variable valve timing mechanism (VTC) for changing the rotation phase of a camshaft with respect to a crankshaft to change at least either valve timing (opening and closing timing) of an intake valve or an exhaust valve, and a control method for the same.
2. Description of Related Art
To change valve timing according to the engine operating conditions, VTC is known in which the rotation phase of the camshaft with respect to the crankshaft is changed by an actuator. In an engine equipped with VTC, there is proposed a technique for changing VTC to valve timing suitable for starting when an engine is stopped to ensure the starting performance of the engine, as described in Japanese Laid-Open (Kokai) Patent Application Publication No. 2009-13975.
In the meantime, VTC attached to an end of the camshaft is subjected to an alternating torque (cam torque), made up of a reaction force of a valve spring and an inertial force by the mass of a movable part, according to the rotation of the camshaft. In a low rpm (revolutions per minute) range of the engine, in which the alternating frequency of the cam torque is equal to or less than the drive frequency of VTC, the reaction force of the valve spring becomes dominant unlike in a high rpm range of the engine, in which the cam torque can be regarded as an average value, to make the cam torque have a substantially sinusoidal waveform. Then, depending on the magnitude of the cam torque, the angle of VTC may vary immediately after engine start, and thus, there is a possibility that the VTC angle suitable for engine start cannot be maintained.